Spectacular growth in mobile phones (cell phones, portable phones) during recent years has encouraged professionals in this field to continue offering ever more new products and services. More especially, the arrival of multimedia has inspired these professionals to integrate numerous applications into mobile phones. These new applications require multiple connectivity to the cellular network besides connectivity to Wireless Personal Area Networks (WPAN), Bluetooth is one example of this technology which has been used as a basis for a new standard—IEEE 802.15.
This being so, consequently and in order to comply with the various standards, the electronics of these new products must be capable of operating over an extremely wide range of frequency bands. By way of example, the following different ranges of frequency bands are encountered among those used in telecommunications:
BandFrequencyGSM/GPRS/EDGE850 MHz, 900 MHz, 1.8 GHz, 1.9 GHzWCDMA2 GHz802.11 a/g2.4 GHz and 5 GHzGPS1.6 GHzUWB3 to 7 GHzRFID2.45 GHzW-CDMA: Wideband Code-Division Multiple AccessGPS: Global Positioning SystemUWB: Ultra Wide Band802.11 a/g: System for wireless networkingRFID: Radio-frequency Identification (especially labels).
As is known, the electrical performance of receivers both in terms of sensitivity and selectivity is chiefly dictated by the frequency synthesiser, i.e. the device in radio-frequency sensors that is used to generate the carrier frequency of the signal. To cover the various frequency ranges mentioned above, multi-standard, multiband devices need to use a large number of radio-frequency oscillators.
Known oscillators include LC resonators which have a quality coefficient or quality factor Q=f/□f that is relatively low (4 to 10 in the frequency band in question). Oscillators made using such a resonator have average performance, especially in terms of spectral purity (phase jitter). In addition, frequency tunability is obtained with the aid of a variable MOS type capacitance (C) and is low, since the frequency variation that can be obtained is of the order of 20% of the carrier frequency value.
Not only this, the frequency bands allocated to telecommunications are becoming increasingly saturated, thus compromising a static allocation concept for said bands. To solve this saturation problem, one solution is to make use of dynamic frequency allocation. This principle relies on the ability to analyse the frequency spectrum and, as far as application to 1 GHz to 10 GHz telecommunications is concerned, to identify unoccupied frequency bands in order to be able to use them. This is referred to as a “radio-opportunistic” system.
However, in order to use this dynamic frequency allocation principle, the devices in question, in this case mobile phones, must have extremely wideband oscillators and offer extremely good performance in terms of phase jitter, and hence a high quality factor. This requirement effectively rules out LC-resonator-based oscillators which would involve using complex, expensive architecture.
One technical solution capable of meeting these requirements can be to use spintronic radio-frequency oscillators. Thus, using such oscillators, it is possible to obtain a wide frequency band with a high quality factor Q and straightforward frequency tunability and, moreover, to use a relatively simple architecture.
Spin electronics uses the spin of electrons as an additional degree of freedom in order to generate new effects.
Spin polarisation of an electric current is a result of asymmetry between the diffusion of “spin-up” type conduction electrons (i.e. parallel to local magnetisation) and that of “spin-down” type conduction electrons (i.e. antiparallel to local magnetisation). This asymmetry results in asymmetrical conductivity between the two spin-up and spin-down channels, and hence net spin polarisation of the current.
This spin polarisation of current causes magnetoresistive phenomena in magnetic multilayers such as giant magnetoresistance (Baibich, M., Broto, J. M., Fert, A., Nguyen Van Dau, F., Petroff, F., Etienne, P., Creuzet, G., Friederch, A. and Chazelas, J., “Giant magnetoresistance of (001)Fe/(001)Cr magnetic superlattices”, Physical Review Letters, Vol. 61 (1988), 2472-5), or tunnel magnetoresistance (Moodera, J. S., Kinder, L. R., Wong, T. M. and Meservey, R., “Large magnetoresistance at room temperature in ferromagnetic thin film tunnel junctions”, Physical Review Letters, Vol. 74 (1995), 3273-6).
In addition, it has also been observed that passing a spin-polarised current through a magnetic thin film can induce reversal of its magnetisation in the absence of any external magnetic field (Katine, J. A., Albert, F. J., Buhrman, R. A., Myers, E. B., and Ralph, D. C., “Current-Driven Magnetization Reversal and Spin-Wave Excitations in Co/Cu/Co Pillars”, Physical Review Letters, Vol. 84 (2000), 3149-52).
The polarised current may also generate sustained magnetic excitation, also referred to as “oscillations” (Kiselev, S. I., Sankey, J. C., Krivorotov, I. N., Emley, N. C., Schoekopf, R. J., Buhrman, R. A., and Ralph, D. C., “Microwave oscillations of a nanomagnet driven by a spin-polarized current”, Nature, Vol. 425 (2003), 380-3).
Using the effect of generating sustained magnetic excitation in a magnetoresistive device makes it possible to convert this effect into electrical resistance modulation that can be directly used in electronic circuits and hence, consequently, is capable of acting directly at the level of frequency.
U.S. Pat. No. 5,695,864 describes various developments that use the physical principle mentioned above. It describes, in particular, precession of the magnetisation of a magnetic layer through which a spin-polarised electric current flows.
The physical principle used will be described below in detail in relation to FIG. 1. In the context of a three-layer magnetic structure, two magnetic layers (1 and 2) are separated by a non-magnetic layer (3) (the term “non-magnetic” is taken to mean diamagnetic or paramagnetic). This intermediate layer (3) is also called a “spacer”. Its thickness is sufficiently small to enable it to transmit a spin-polarised current and sufficiently large to ensure magnetic decoupling between layers (1 and 2) which it separates.
Layer (1) is a so-called “anchored” ferromagnetic layer in the sense that it has a fixed magnetisation direction. Generally speaking, this layer (1) is coupled to an antiferromagnetic layer, the function of which is to anchor said layer (1) so that its magnetisation does not flip when the assembly is subjected to an electric current. This layer (1) may also be made up of several layers as, for example, described in U.S. Pat. No. 5,883,725, in order to build a so-called “synthetic antiferromagnetic” layer. This layer (1) is called the “polariser”. In fact, because of its fixed magnetisation direction, it induces spin polarisation of the electric current that flows through it. As already stated, in a magnetic material, the conductivity of elections that have spin parallel to the local magnetisation (spin-up) is different to that of electrons that have opposite spin (spin-down). Thus, reflection and transmission at the interface between the two layers having different magnetic properties are phenomena that depend on spin. The conduction electrons that reach the interface between layer (1) and spacer (3) mostly have a spin type (up or down) that depends on the nature of the materials used.
For layer (1) (polariser), one selects either a ferromagnetic layer of sufficient thickness to ensure maximum polarisation of the current or a “synthetic antiferromagnetic” (SAF) layer of appropriate thickness to achieve this same objective. With a transport geometry that is perpendicular to the plane of the layers, it is known that the characteristic length is the so-called spin diffusion length (Valet, T. and Fert, A., “Theory of perpendicular magnetoresistance in magnetic multilayers”, Physical Review B, Vol. 48 (1993), 7099-7113). The term “sufficient thickness”, in respect of the polarisation layer, is therefore taken to mean a thickness that is sufficiently large relative to this spin diffusion length (typically 5 nm in Ni80Fe20 at ambient temperature). Obviously, the polarisation layer may consist of one or more layers (for example a NiFe/CoFe bilayer or a multilayer laminated composite (CoFe1 nm/Cu0.3 nM)3/CoFe1 nm) in order to encourage polarisation of the current or shorten the spin diffusion length.
If the thickness of spacer (3) is sufficiently small, polarisation of the electric current that flows through the layers at right angles is almost entirely preserved until it reaches the interface between spacer (3) and layer (2). This layer (2) is a magnetically soft so-called “free” layer, e.g. the direction of its magnetisation can easily be changed by the effect of a weak external field (typically a layer made of Ni80Fe20 Permalloy or CoFe alloys or formed by associating two layers such as NiFe/CoFe).
At the level of the interface between layer (2) and layer (3), spin transfer occurs between the spin-polarised current and the magnetic moment of layer (2). If the latter and the spin polarisation direction (imparted by the magnetisation of layer (1)) are not collinear, the current affects the magnetisation of layer (2) enough to make it rotate (precession). The sign of the spin transfer torque depends on the direction of the applied current:                If the conduction electrons move from polariser (1) to layer (2), the spin transfer torque will orientate the magnetisation of said layer (2) parallel to that of layer (1);        In contrast, if the conduction electrons move from layer (2) to polariser (1), said torque will orientate the magnetisation of layer (2) antiparallel to that of layer (1).        
It has been demonstrated that, depending on the amplitude of the current or even the external magnetic field applied, two distinct effects can be detected:                Firstly, reversal of the magnetisation of layer (2); this reversal may be hysteretic or reversible, depending on the amplitude of the current or even the external magnetic field; this phenomenon can also be used as a means of writing information in the context of producing random-access memories, also referred to as MRAMs;        Also excitation of the sustained precession states of the magnetic moment of layer (2): this is the effect that is exploited within the framework of the present invention.        
When one considers the sustained precession of the magnetic moment of layer (2), several modes have been revealed by microwave frequency measurements, depending on the relative intensity of the applied electric current in particular:                mode A: small-angle precession of the ferromagnetic resonance (FMR) type: this precession mode occurs for relatively weak intensity currents and is characterised by signals having a given frequency that does not depend on the applied current;        mode B: large-angle precession: this precession mode occurs if the applied current is increased above a certain threshold and is characterised by marked frequency dependence on the applied current;        mode C: microwave RTS noise for medium-intensity currents besides weak magnetic fields. The spectra measured under these conditions show very wide, very high-amplitude peaks centred around 1 GHz.        
In the context of the present invention, the behaviours exploited are those for which the precession frequency can be adjusted, either by influencing the current or, preferably, by influencing both the current and the external magnetic field. Such structures on nanostructures are integrated in magnetoresistive assemblies or devices. In the case of both giant magnetoresistance (GMR) in metallic systems or tunnel magnetoresistance (TMR) in metal-insulator-magnetic-metal tunnel junctions, magnetisation precession results in variation of the electrical resistance measured when a current is applied in a direction that is perpendicular to the plane of the layers (CCP or Current Perpendicular to Plane geometry).
Without going into details that are deemed to be known by those skilled in the art, magnetic tunnel junctions referred to as TMRs or MTJs, at their simplest, consist of two magnetic layers, it being possible to vary the relative orientation of their magnetisation and the layers being separated by an insulating layer.
The magnetoresistive devices used employ stacks made in two different ways:                so-called “point contact” stacks in which the active layers (layer 1, layer 2, layer 3) are not etched using nanometric patterns, or if they are, are then fabricated using very large patterns (roughly μm2); an extremely narrow metallic contact, typically 50 nm above layer (2), is produced by means of an external nanotip (for example tip of an atomic force microscope) or internal nanotip (screen printed pillar).        “pillar” type stacks: all the layers are etched to fabricate a pillar having a diameter of the order of 100 nm; in order to prevent the occurrence of significant magnetostatic interaction between layers (1) and (2), layer (1) is sometimes left unetched.        
When current is passed through the first type of device perpendicular to the plane of the layers, the current lines all converge towards the nanocontact (the point contact) and diverge towards the inside of the stack in a cone, the shape of which depends on the electrical resistivity of the various layers. In the second case, with a pillar geometry, the current flows more or less uniformly over the entire cross-section of the pillar.
It has been demonstrated, with the aid of micromagnetic simulations, that the so-called “point contact” method is more advantageous for fabricating radio-frequency oscillators insofar as it minimises the occurrence of incoherent excitation produced by edge effects. FIG. 2 (pillar) and FIG. 3 (point contact) show these two types of stacks.
In relation to these structures, FR 2 817 999 states that when the polariser (layer 1) is magnetised in a direction that is perpendicular to the plane of the layers that make up the magnetoresistive device, and the moment of layer (2) is oriented in a direction parallel to the interfaces, the critical current required to induce precession of said magnetisation can be reduced.
Although, at a theoretical level, the magnetoresistive devices thus described make it possible to achieve implementation of radio-frequency oscillators that satisfy industrial manufacturing requirements (wide frequency range, dynamic frequency allocation is possible, high quality factor Q), it is nevertheless apparent that the actual quality of these devices depends on the consistency of the magnetisation precession produced by the electric current that flows through the layers.
The term “consistency of magnetisation precession” denotes the fact that magnetisation is moved as a single unit over the entire extent of the current sheet through the structure (i.e. over the cross-section of the pillar with a pillar geometry and over the cross-section of the current cone at the level of the free layer if there is a nanocontact) in contrast to producing multiple small excitations that are inconsistent.
Thus, greater consistency results in oscillation signals of narrower frequency and lower amplitude: it is the object of this invention to propose means of increasing the consistency of the dynamic movement of magnetisation precession. Because a reduction in amplitude is not the sought-after effect, once frequency narrowness has been obtained, attempts will be made to boost the amplitude.